As of yesterday, approaching The Drifter (after having completed the first few steps along the Gambit Prime and Reckoning quests) unlocks a decision point, giving you the option to “turn your back on the Vanguard to stand with the Drifter.”
Bungie says that your decision will have consequences, which for now appear to be limited to “different quest content,” and “different rewards” depending on which route you pick, although siding with The Drifter over The Vanguard will also limit access to a particular lore book. In the long-term, however, it looks as though plenty more is set to change. In a notably Telltale-style take on storytelling, you’re told that “The Drifter will remember your choice,” and that “this choice may impact future content for [your current] character.”